Concrete is strong one way and weak the other
Plain concrete handles compression well, weight pressing straight down on it, but it is weak in tension, the pulling and bending force that shows up wherever a slab or a wall spans a soft spot or gets loaded unevenly. Steel is strong in tension in a way concrete is not, so bar tied into a grid and set inside the concrete gives the pour the strength it is missing on its own. A footing bridging a soft spot in the ground, or a wall holding back the weight of backfilled soil, is doing tension work, and that is exactly where steel earns its place rather than being optional.
A grid, tied and lapped, not loose sticks in the mix
Rebar goes in as a planned grid, not scattered pieces. Bar runs continuous where it can, gets lapped a set distance at the splices where one length ends and the next begins, and turns corners with bent bar instead of two straight pieces stopping short of each other. All of it gets tied together so it holds its shape and its spacing once concrete starts landing on it, and held up off the ground on chairs so it ends up buried in the middle of the pour rather than sitting at the bottom of the form.
Not the same thing as wire mesh
Rebar and wire mesh both count as reinforcing steel, and that is usually where the two get confused. Rebar is individual bar, tied into a grid sized and spaced to a structural drawing, and it is standard on footings, foundation walls and anywhere a pour is doing real structural work. Wire mesh is a factory made grid in sheets or rolls, used differently and for different reasons on flatwork. Which one, or whether a job needs both, comes off the plan and what the concrete has to carry.
What it adds to a job
Rebar is priced as its own add on our estimator, $0.75 to $1.25 per square foot, on top of the base rate for the concrete itself. Whether a job needs it, how much, and what size and spacing, comes off the structural drawings where there is a plan, and off what the concrete has to carry where there is not. Foundation work and poured walls carry steel as a matter of course. Some flatwork does too, depending on what will sit or drive on it.